python - 上下文管理器适合这项工作吗？

Question

下面粘贴的代码执行以下操作:

• 创建一个导入 Hook
• 创建一个设置 meta_path 并在退出时清理的上下文管理器。
• 在 imports.log 中转储通过输入传递的程序完成的所有导入

现在我想知道在这种情况下使用上下文管理器是否是个好主意，因为实际上我没有标准的 try/finally 流程，而只是设置和清理。

另一件事——这一行:

with CollectorContext(cl, sys.argv, 'imports.log') as cc:

cc 是否变成了None？它不应该是一个 CollectorContext 对象吗？

from __future__ import with_statement
import os
import sys

class CollectImports(object):
    """
    Import hook, adds each import request to the loaded set and dumps
    them to file
    """

    def __init__(self):
        self.loaded = set()

    def __str__(self):
        return str(self.loaded)

    def dump_to_file(self, fname):
        """Dump the loaded set to file
        """
        dumped_str = '\n'.join(x for x in self.loaded)
        open(fname, 'w').write(dumped_str)

    def find_module(self, module_name, package=None):
        self.loaded.add(module_name)


class CollectorContext(object):
    """Sets the meta_path hook with the passed import hook when
    entering and clean up when exiting
    """

    def __init__(self, collector, argv, output_file):
        self.collector = collector
        self.argv = argv
        self.output_file = output_file

    def __enter__(self):
        self.argv = self.argv[1:]
        sys.meta_path.append(self.collector)

    def __exit__(self, type, value, traceback):
        # TODO: should assert that the variables are None, otherwise
        # we are quitting with some exceptions
        self.collector.dump_to_file(self.output_file)
        sys.meta_path.remove(self.collector)


def main_context():
    cl = CollectImports()

    with CollectorContext(cl, sys.argv, 'imports.log') as cc:
        progname = sys.argv[0]
        code = compile(open(progname).read(), progname, 'exec')
        exec(code)


if __name__ == '__main__':
    sys.argv = sys.argv[1:]
    main_context()

Answer 1

我觉得这个概念没问题。同样，我看不出有任何理由反对在 finally: 子句中进行清理，因此上下文管理器非常适合。

你的 cc 是 None，因为你告诉它如此。

如果你不想那样，改变你的__enter__ method to return something else :

The value returned by this method is bound to the identifier in the as clause of with statements using this context manager.

def __enter__(self):
    self.argv = self.argv[1:]
    sys.meta_path.append(self.collector)
    return self
    # or
    return self.collector
    # or
    return "I don't know what to return here"

然后

with CollectorContext(cl, sys.argv, 'imports.log') as cc:
    print cc, repr(cc) # there you see what happens.
    progname = sys.argv[0]
    code = compile(open(progname).read(), progname, 'exec')
    exec(code)